Numerical evolutions of a black hole-neutron star system in full General Relativity: I. Head-on collision

TL;DR

This paper presents the first full General Relativity simulations of head-on collisions between a black hole and a neutron star of comparable mass, revealing rapid accretion and gravitational wave emission.

Contribution

It introduces a novel simulation approach that handles arbitrary mass ratios and black hole positions without limitations, advancing the modeling of such extreme astrophysical events.

Findings

Neutron star is promptly and fully accreted into the black hole.

The collision completes before approximately 300M, with evolution observed up to 1700M.

First estimate of gravitational-wave radiative efficiency for this system.

Abstract

We present the first simulations in full General Relativity of the head-on collision between a neutron star and a black hole of comparable mass. These simulations are performed through the solution of the Einstein equations combined with an accurate solution of the relativistic hydrodynamics equations via high-resolution shock-capturing techniques. The initial data is obtained by following the York-Lichnerowicz conformal decomposition with the assumption of time symmetry. Unlike other relativistic studies of such systems, no limitation is set for the mass ratio between the black hole and the neutron star, nor on the position of the black hole, whose apparent horizon is entirely contained within the computational domain. The latter extends over ~400M and is covered with six levels of fixed mesh refinement. Concentrating on a prototypical binary system with mass ratio ~6, we find that although a tidal deformation is evident the neutron star is accreted promptly and entirely into the black hole. While the collision is completed before ~300M, the evolution is carried over up to ~1700M, thus providing time for the extraction of the gravitational-wave signal produced and allowing for a first estimate of the radiative efficiency of processes of this type.